1. Field of the Invention
The present invention relates to a process for producing lactide and a process for producing polylactic acid from fermented lactic acid employed as a starting material, and to a process for producing lactide and a process for producing polylactic acid from ammonium lactate obtained by lactic fermentation. Also, the present invention relates to a process for producing lactate ester from ammonium lactate obtained by lactic fermentation.
Lactic acid includes L-lactic acid, D-lactic acid, and a mixture of L-lactic acid and D-lactic acid. Lactide is a cyclic dimer of the lactic acid, and lactide is particularly useful as a starting material for producing polylactic acid. The polylactic acid is a polymer in which the lactic acid is contained as a major constitutional monomer, and the polylactic acid includes not only a homopolymer of the lactic acid but also a copolymer of the lactic acid with other monomers. The polylactic acid is useful as a biodegradable plastic derived from biomass employed as a starting material.
2. Description of the Related Art
Traditionally, the production of polylactic acid has been performed by a dehydration condensation directly from lactic acid, a dealcohol condensation from lactate ester, or a ring-opening polymerization of lactide.
With regard to the production of polylactic acid from lactate ester, for example, the specification of U.S. Pat. No. 2,534,255 discloses a method producing hydroxy polyester from hydroxycarboxylate ester in the presence of a catalyst for esterification.
Japanese Laid-open Patent Publication No. H07-173264 (1995) discloses a method condensing lactate ester to produce polylactic acid with a weight-average molecular weight of 15,000 or above. There is no description in which lactide is synthesized from said polymerized product. It is necessary to consume great time for producing the polylactic acid with a weight-average molecular weight of 15,000 or above.
Japanese Patent Publication (B2) No. 3024907 discloses a process polycondensing lactate ester in a batch type reactor while removing generated alcohol, and further polycondensing the obtained oligomer in a screw type extruding machine while removing generated alcohol so as to produce polylactic acid from the lactate ester. There is no description in which lactide is synthesized from said polymerized product.
Japanese Laid-open Patent Publication No. H10-36366 (1998) discloses a process synthesizing lactate ester from synthesized lactic acid (DL-lactic acid), dealcoholizing the lactate ester to synthesize a lactic acid oligomer, and subsequently synthesizing lactide.
Japanese Laid-open Patent Publication No. H11-209370 (1999) discloses a process for producing lactide, which comprises dealcoholizing lactate ester in the presence of a monobutyltin compound to produce a composition containing polylactic acid or a composition containing polylactic acid and lactide, and then heating the composition in the presence of a monobutyltin compound.
Besides, it has been previously known that ammonium lactate is directly esterified with alcohol (in INDUSTRIAL AND ENGINEERING CHEMISTRY, VOL.44, NO.9, 2189-2191, September 1957).
Japanese Laid-open Patent Publication No. H06-311886 (1994) discloses a process adding butanol or pentanol to ammonium lactate generated by fermentation, and adding mineral acid thereto at the later stage of the reaction to proceed reaction further, thereby recovering lactate ester with recovering ammonia.
Furthermore, Japanese Patent Publication (B2) No. 2830896 discloses a process synthesizing an oligomer from lactic acid used as a starting material, refluxing the oligomer to synthesize an oligomer with higher degree of polymerization, and producing lactide from the oligomer with higher degree of polymerization. There is no disclosure of a process in which polycondensation is conducted employing lactate ester while refluxing a monomer of the lactate ester and an oligomer thereof.
Thus, a process which comprises synthesizing a lactic acid oligomer in which lactate ester derived from fermented lactic acid is employed as a starting material, and depolymerizing the lactic acid oligomer to produce lactide, and a process which comprises synthesizing a lactic acid oligomer in which lactate ester derived from fermented lactic acid is employed as a starting material, depolymerizing the lactic acid oligomer to produce lactide, and producing polylactic acid from the lactide, have never been known.
Further, in a synthesis of lactate ester from lactic fermentation, producing lactic acid and ethanol by means of hetero-type lactic fermentation while neutralizing with ammonia, and esterifying this lactic acid, have never been known. Hetero-lactic acid bacteria produce lactic acid and ethanol from glucose, and produce lactic acid and acetic acid from xylose.
Hitherto, in producing lactic acid, lactic acid was obtained by steps of performing lactic fermentation of hexose (glucose) or sucrose (consisting of glucose and fructose); neutralizing with calcium carbonate; concentrating to allow calcium lactate to precipitate; liberating with sulfuric acid to produce crude lactic acid; further converting into ester with methanol or ethanol; and distilling the ester followed by hydrolyzing the ester to produce lactic acid. According to this process, there were problems in which significant amounts of calcium sulfate were generated as a byproduct.
Accordingly, a process which comprises neutralizing with ammonia water or ammonia during lactic fermentation to obtain ammonium lactate, converting ammonium lactate into ester with alcohol followed by distilling, recovering ammonia, and hydrolyzing lactate ester, is developed.
Hitherto, there was no consistent process for producing polylactic acid from biomass. By completing the consistent process, cost for producing polylactic acid can be reduced. Namely, there were lots of useless steps conducted, because the production of polylactic acid was formerly considered to be separated from the production of lactic acid. Concretely, the useless step means a step of hydrolyzing lactate ester to produce lactic acid. Besides, by synthesizing polylactic acid from lactate ester, corrosive resistance required for a reactor can be mitigated, and significant contribution in a capital investment can be achieved. Because lactic acid has corrosive properties, especially in the case of concentrating or synthesizing lactide at a temperature exceeding 140xc2x0 C., it has been necessary to use a reactor and a tube made of titanium or tantalum, or a reactor and a tube with glass lining. However, in the case of using lactate ester, it is not necessary to use a material with corrosive resistance for a reactor, thereby achieving drastic cost reduction in equipments of facilities.
In particular, there is a worldwide subject of how to utilize wood type biomass at present. In Japan, there is a subject of how to treat wastes such as old papers, construction waste materials, lumbers from thinning, rice straws and the others. In overseas, there is a stream to use wood materials as a starting material for fermentation because wood materials cannot be directly used as foodstuffs for people.
The wood type biomass includes cellulose and hemicellulose. As a method for saccharification of cellulose and hemicellulose, a sulfuric acid method, a high-pressure hot water method, and an enzyme method are in a stage of practical use. Sugar obtained by saccharification is hexose and pentose. The hexose is mainly glucose, and the pentose is mainly xylose.
By homo-type lactic fermentation, 1 mol of glucose is converted to 2 mols of lactic acid, which does not waste carbon atoms. Further, among the lactic acid bacteria, there are strains capable of producing 5 mols of lactic acid from 3 mol of xylose. Furthermore, by hetero-type lactic fermentation, 1 mol of glucose is converted to 1 mol of lactic acid and 1 mol of ethanol (in this case, 1 mol of carbon dioxide gas is generated). Namely, glucose is a particularly suitable starting material for synthesizing lactate ester.
On the other hand, by hetero-type lactic fermentation, 1 mol of lactic acid and 1 mol of acetic acid are produced from 1 mol of xylose. Thus, in the present invention, hetero-lactic fermentation is conducted while neutralizing with ammonia to generate lactic acid and acetic acid, and generated lactic acid and acetic acid are esterified with alcohol (ethanol) to synthesize esters that are isolated from a fermentation liquid as lactate ester and acetate ester, respectively.
In the case of allowing ammonium lactate to react with ethanol to produce ethyl lactate, there were problems in which yield is low and reaction slowly proceeds due to low boiling point of ethanol that is 78xc2x0 C.
In the case of allowing ammonium lactate to react with hydrophobic alcohol such as butanol to produce lactate ester, there were problems in which yield is low in approximately 50% even if water is removed by means of a separator.
An object of the present invention is to provide a process for consistently producing lactide from ammonium lactate obtained by lactic fermentation, and a process for consistently producing polylactic acid from ammonium lactate obtained by lactic fermentation. Further, an object of the present invention is to provide a process for producing lactate ester from ammonium lactate obtained by lactic fermentation.
The present invention is a process for producing lactide, which comprises the steps of: (1) synthesizing lactate ester from ammonium lactate obtained by lactic fermentation; (2) polycondensing the lactate ester in the presence of a catalyst other than monobutyltin, whereby polylactic acid with a weight-average molecular weight of less than 15,000 (lactic acid prepolymer) is synthesized; and (3) depolymerizing the polylactic acid, whereby lactide is produced.
The present invention is said process for producing lactide, wherein the step (1) comprises: allowing the ammonium lactate obtained by lactic fermentation to react with alcohol, whereby lactate ester is synthesized; and recovering ammonia.
The present invention is said process for producing lactide, wherein the step (1) comprises: allowing the ammonium lactate obtained by lactic fermentation to react with alcohol other than alcohol of which the number of carbon atoms is 4 or 5, whereby lactate ester is synthesized; and recovering ammonia.
The present invention is said process for producing lactide, wherein the step (1) comprises: allowing the ammonium lactate obtained by lactic fermentation to react with ethanol under a pressurized condition, whereby ethyl lactate is synthesized; and recovering ammonia.
The present invention is said process for producing lactide, wherein the step (1) comprises: allowing an aqueous solution of the ammonium lactate obtained by lactic fermentation to react with alcohol of which the number of carbon atoms is 4 or more in a reactor under reflux, in which the alcohol is separable from water to form two layers with water;
trapping at least a portion of condensed water and at least a portion of condensed alcohol in a separator; discharging at least a portion of the water trapped in the separator as a lower layer; adding fresh water to the separator and returning at least a portion of the alcohol trapped in the separator as an upper layer into the reactor; and repeating this series of the procedures,
whereby lactate ester is synthesized; and recovering ammonia.
The present invention is said process for producing lactide, wherein the step (1) comprises: allowing an aqueous solution of the ammonium lactate obtained by lactic fermentation to react with alcohol of which the number of carbon atoms is 4 or more in a reactor under reflux, in which the alcohol is separable from water to form two layers with water; distilling off a portion of the alcohol or the whole alcohol continuously; and adding fresh alcohol to the reactor, whereby lactate ester is synthesized; and recovering ammonia.
The present invention is said process for producing lactide, wherein the step (1) comprises: allowing the ammonium lactate obtained by lactic fermentation to react with ethanol; after completing the reaction, distilling off ethanol and ethyl lactate, respectively;
adding fresh ethanol to a remained solution of the ammonium lactate or adding fresh ethanol and fresh ammonium lactate obtained by lactic fermentation to the remained solution of the ammonium lactate; allowing the ammonium lactate to react with ethanol again; and repeating this reaction procedure, whereby ethyl lactate is synthesized; and recovering ammonia.
The present invention is said process for producing lactide, wherein the polycondensation in the step (2) is conducted while refluxing the lactate ester and/or the polylactic acid (lactic acid prepolymer) which is a polycondensed product of the lactate ester.
The present invention is said process for producing lactide, wherein the step (2) comprises: polycondensing the lactate ester, whereby the lactic acid prepolymer is obtained; and recovering generated alcohol.
The present invention is said process for producing lactide, wherein the step (2) comprises raising a temperature of a reactor continuously in the step of polycondensing the lactate ester.
The present invention is said process for producing lactide, wherein the step (1) comprises: culturing a microorganism capable of conducting hetero-lactic fermentation in a culture medium containing hexose while controlling a pH value with ammonia to produce the ammonium lactate and ethanol; and
allowing the ammonium lactate to react with alcohol, whereby lactate ester is synthesized; and recovering ammonia.
The present invention is said process for producing lactide, wherein the step (1) comprises: culturing a microorganism capable of conducting hetero-lactic fermentation in a culture medium containing hexose while controlling a pH value with ammonia to produce the ammonium lactate and ethanol; and
allowing the ammonium lactate to react with alcohol other than alcohol of which the number of carbon atoms is 4 or 5, whereby lactate ester is synthesized; and recovering ammonia.
The present invention is said process for producing lactide, wherein the step (1) comprises: culturing a microorganism capable of conducting hetero-lactic fermentation in a culture medium containing pentose while controlling a pH value with ammonia to produce ammonium lactate and ammonium acetate;
allowing the ammonium lactate and the ammonium acetate to react with ethanol, whereby ethyl lactate and ethyl acetate are synthesized, respectively; recovering ammonia; and further removing ethyl acetate by distillation; and
wherein the step (2) comprises: polycondensing the ethyl lactate, whereby the lactic acid prepolymer is obtained; and recovering ethanol.
The present invention is said process for producing lactide, wherein the step (1) comprises: culturing a microorganism capable of conducting hetero-lactic fermentation in a culture medium containing pentose and hexose while controlling a pH value with ammonia to produce ammonium lactate, ethanol and ammonium acetate;
allowing the ammonium lactate and the ammonium acetate to react with ethanol, whereby ethyl lactate and ethyl acetate are synthesized, respectively; recovering ammonia; and further removing the ethyl acetate by distillation; and
wherein the step (2) comprises: polycondensing the ethyl lactate, whereby the lactic acid prepolymer is obtained; and recovering ethanol.
Also, the present invention is a process for producing polylactic acid, which comprises ring-opening polycondensing lactide obtained by any one of said process, whereby polylactic acid is obtained.
Further, the present invention is a process for producing ethyl lactate, which comprises allowing ammonium lactate obtained by lactic fermentation to react with ethanol under a pressurized condition, whereby ethyl lactate is synthesized.
The present invention is a process for producing lactate ester, which comprises: allowing an aqueous solution of ammonium lactate obtained by lactic fermentation to react with alcohol of which the number of carbon atoms is 4 or more in a reactor under reflux, in which the alcohol is separable from water to form two layers with water;
trapping at least a portion of condensed water and at least a portion of condensed alcohol in a separator; discharging at least a portion of the water trapped in the separator as a lower layer; adding fresh water to the separator and returning at least a portion of the alcohol trapped in the separator as an upper layer into the reactor; and repeating this series of the procedures,
whereby lactate ester is synthesized.
The present invention is a process for producing lactate ester, which comprises: allowing an aqueous solution of ammonium lactate obtained by lactic fermentation to react with alcohol of which the number of carbon atoms is 4 or more in a reactor under reflux, in which the alcohol is separable from water to form two layers with water; distilling off a portion of the alcohol or the whole alcohol continuously; and adding fresh alcohol to the reactor, whereby lactate ester is synthesized.
The present invention is a process for producing lactic acid prepolymer, which comprises the steps of: (1) synthesizing lactate ester from ammonium lactate obtained by lactic fermentation; and (2) polycondensing the lactate ester in the presence of a catalyst other than monobutyltin, in which said polycondensation is conducted while refluxing the lactate ester and/or polylactic acid (lactic acid prepolymer) which is a polycondensed product of the lactate ester, whereby polylactic acid with a weight-average molecular weight of less than 15,000 (lactic acid prepolymer) is synthesized.
In the case of allowing ammonium lactate to react with ethanol to produce lactate ester, a method by adding, for example, 5 mols to 20 mols of ethanol based on 1 mol of ammonium lactate to shift equilibrium to the side of the ester can be considered.
Further, recovery yield of ethyl lactate can be increased by distilling off ethanol and generated ethyl lactate followed by adding a fresh lactic fermentation liquid and ethanol into the unreacted ammonium lactate remained in a reactor so as to perform reaction.
In this reaction, a reaction rate can be increased by pressurizing to raise a vapor pressure of ethanol and to raise a reaction temperature, because a boiling point of ethanol, 78xc2x0 C., is low. Also, in this case, eliminated ammonia is removed to the outside of the system via a valve for controlling pressure. As a method of pressurizing, a method by utilizing a vapor pressure of ethanol it self and a method for pressurizing by an inert gas such as nitrogen gas are preferable.
In a method allowing ammonium lactate to react with butanol under reflux, if ammonia is saturated in water separated in a separator by vaporizing from a reactor followed by condensing, ammonia would be accumulated in butanol trapped in the separator as an upper layer, thereby becoming the reaction slow. Namely, it is considered that water trapped in the separator as a lower layer performs work for extracting ammonia existing in the upper layer butanol into the water side so as to reduce an amount of ammonia existing in butanol to be returned into the reactor. Consequently, by replacing the lower layer water trapped in the separator with fresh water, ammonia in the upper layer butanol is extracted with this fresh water. And, by returning butanol in which the ammonia content is reduced into the reactor, the reaction proceeds further.
According to the present invention, a process for consistently producing lactide from ammonium lactate obtained by lactic fermentation, and a process for consistently producing polylactic acid from ammonium lactate obtained by lactic fermentation, are provided. Further, according to the present invention, a process for producing lactate ester from ammonium lactate obtained by lactic fermentation is provided.
In the present process, ammonium lactate from lactic fermentation is esterified as it is, thereby eliminating the need of a procedure in which lactate salt is converted to a free form of lactic acid as former processes required. Consequently, steps in producing lactide and producing polylactic acid can be reduced, and lactide and polylactic acid can be produced in less expensive manner.
Furthermore, in the present process, neutral ammonium lactate is employed as a starting material, thereby eliminating the need to use a reactor and tube with acid resistance properties such as those made of titanium or glass. Therefore, the present process is economic because capital investment for production can be reduced, comparing with former methods that required a reactor and tube with acid resistance properties because a free form of lactic acid is used as a starting material.